1. Field of the Invention
The present invention relates to a dynamic pressure-type hydrodynamic bearing device, as well as a spindle motor and information device using the same.
2. Description of the Prior Art
A hydrodynamic bearing device comprises a shaft and a sleeve that supports the shaft, and a lubricant that is interposed in the gap between the two parts. With rotation of the shaft, the lubricant is gathered up by dynamic pressure-generating grooves that are formed on the shaft or sleeve, and generates pressure such that the shaft is supported within the sleeve without coming into contact therewith. As a result, when high-speed rotation is attained, ambient noise during the rotation can be alleviated.
A spindle motor equipped with such a hydrodynamic bearing device can provide the requisite rotational accuracy with an increased recording density of the medium, and can furthermore provide excellent shock resistance and quietness. Thus, it can be used in a majority of motors for application in such representative magnetic disk devices as information technology equipment and audio-visual equipment.
In recent years, the demand has grown stronger for magnetic disk devices that are increasingly miniaturized and more energy-conserving, for decreased power consumption for the spindle motor that is the main component, and in particular, for reduced torque in the hydrodynamic bearing device that exerts a significant influence on decreasing power consumption in the motor. Since the torque of the hydrodynamic bearing device will be roughly proportional to the viscosity of the lubricant used to fill the device, using a lower viscosity lubricant is an effective way to reduce the torque.
For this reason, esters such as dioctyl sebacate (DOS), dioctyl azelate (DOZ), and dioctyl adipate (DOA) have been proposed for use as lubricants in hydrodynamic bearing devices. Moreover, esters obtained from neopentyl glycol and C6 to C12 monovalent fatty acids and/or their derivatives for use as lubricants in hydrodynamic bearing devices (see for example Japanese published unexamined application No. 2001-316687), the use of esters represented by the generic formula R1—COO-(AO)n—R2 as lubricants for bearings (see for example Japanese published unexamined application No. 2002-206094), and ethers containing viscosity index improvers and anti-wear agents (see for example Japanese published unexamined application No. 2002-348586), have been proposed.
However, while it is possible to reduce the torque in such conventional hydrodynamic bearing devices, since the heat resistance of the lubricant is low (vapor pressure is high), the amount of evaporation will be significant when used over a long period, and it will not be possible to maintain the quantity of lubricant required for stabilized rotation of the bearing device. Consequently, there will be problems with the device having inadequate reliability and the operational lifetime will be shorter.
As a countermeasure to the amount of evaporation, one can consider a method by which the above requirement is addressed by adding an excess of the lubricant. However, this approach will entail problems in that this additional amount can increase the torque and bring a higher cost, and accommodating the additional space will make miniaturization more difficult.
Moreover, resins can undergo dissolution or swelling from coming into contact with ester-type lubricants, leading to the deterioration and lower performance of the resins used in the bearing components or in the material of the seals. This limits the choice of resins that can be used.
Moreover, Japanese published unexamined application No. 2002-348586 teaches that when polymeric viscosity index improvers are added to the lubricant, the molecular bonds of the polymer are cleaved by shearing forces after long-term use at high speed rotations, which will cause marked changes in the viscosity, and raises concerns that the reliability of the hydrodynamic bearing will be impaired.